"No inherent effect. Many Mesmer skills, especially spells which deceive your foes and hinder their movement and attacks, become more effective with higher Illusion Magic."
Illusion Magic is a mixed batch of skills, many with very odd effects. This makes it difficult to give a blanket description for the general effects that all Illusion skills provide. However, Illusion Magic skills generally focus on either layering effects on foes to deal medium amounts of damage as well as hinder their abilities, or creating illusions to protect and heal the caster.
This layering of effects is most effective when used on melee attackers. For example, a combination of Ethereal Burden to slow down foes, and Soothing Images to prevent Warriors from gaining adrenaline, makes it extremely difficult for any melee attacker to hurt you at all. Meanwhile, repeated uses of Phantom Pain (reduces maximum health), Conjure Phantasm (health degeneration) and Clumsiness (damages foe if they attack) can whittle down their health while they remain helpless.
The second focus of Illusion Magic is aiding the caster. A number of skills create illusions that either trick your foe to your own benefit, or provide some sort of benefit to you. Distortion (caster blocks attacks) makes it even more difficult for melee attackers to hurt you, and Illusion of Haste can provide you with the speed you need to escape. Illusion of Weakness (lose health; when health drops below 25%, gain that amount of health back), while appearing useless at first glance, is in fact a fantastic healing spell. Simply activate it, heal yourself through other means (perhaps Ether Feast), then relax, knowing that as soon as your health dips below the danger point, you will be healed by a large amount.
Choosing to focus on Illusion skills from the start may prove difficult, partially because many Illusion skills are Elite skills, and are therefore difficult to acquire and impossible to use in tandem. Consider starting a Mesmer with a focus in Domination Magic, then switching over to Illusion once you have collected the skills and equipment you need for your build.
In conclusion, Illusion Magic may be too strange for newcomers to the game to fully appreciate its myriad of effects, but in the hands of an experienced player, it can be a crippling and powerful tool, especially against melee attackers.